I just installed a Yaesu ATAS-120A autotuning screwdriver antenna to go with the FT-857 in my Smart car. The ATAS is small, light, and automatically tunes quickly to any band from 40 to 6 meters. The antenna is mounted at the roof line with the top of the whip 10' above ground.

Preliminary measurements based on VSWR bandwidth show a radiation efficiency of 11% at 7 MHz in this installation. Simulation using a coil Q of 90 (from the K6STI coil program) and a ground loss of 10 ohms shows a radiation efficiency of 9%.

Interestingly the same simulation using a coil Q of 300 (Little Tarheel II antenna) shows a radiation efficiency of 16% which is 2.5 dB better than the ATAS-120A at 7 MHz.

I'll report back when I refine these measurements and will add field strength measurements against two other mobile antennas.

I don't know where your figures came from, but the ATAS has an efficiency of just about 1% on 40 meters. The coil Q is sub 50 on 40 meters, and not much better even on 10 meters. Every antenna shootout ever conducted where an ATAS was measured, the ATAS was 15 to 25 dB down from the crowd. And, unless you choke it off, the ATAS suffers from common mode.

Comparing one to a Lil Tarheel isn't saying much, except their Q is a bit higher.

Operating mobile on 40 meters and lower presents a challenge for any antenna. The best mobile antenna I ever used on 40 and below was the original, old, big, ugly, "real", Texas Bug Catcher. There may be other antennas that are better, but this is the best one I ever used.

All these motorized, remotely tuned, 5-6 foot long, mobile HF antenna performance claims never stop amusing me. I am sure you know, but just because one can adjust the SWR to a low value, does not mean the antenna is radiating a decent signal.

Dick you are right. To get 'size' with this installation the ATAS-120A is installed at the car roof line. The vertical length of the car becomes part of the 'antenna' and the 64" ATAS-120A becomes just one part of a 120" 'antenna.'

The simulated radiation resistance of the ATAS ground mounted is 1 ohm. But installed as it is the 'antenna' has a radiation resistance of 3.5 ohms. It turns out that the ATAS has an internal base matching inductor and the input resistance at resonance cannot be used to determine the system losses. However, a 3:1 VSWR bandwidth measurement can be used to determine system losses and efficiency.

Testing so far - other than the VSWR bandwidth measurement - is purely anecdotal. Since installation yesterday it's worked Europe short path on 20 meters, long path on 30 meters, and a few stateside stations on 40 meters with easy answers to my CQs.

I don't know where your figures came from, but the ATAS has an efficiency of just about 1% on 40 meters.

I explained where my figures come from.

It isn't the length of the what we call the mobile antenna that determines the radiation resistance, it is the length of the actual antenna. That is measured from the bottom of the vehicle to the top of the mobile antenna. In my installation that is 10' and the radiation resistance at 7 MHz is 3.5 ohms. If the efficiency was 1% the loss resistance would have to be 350 ohms, which it is not.

A 10' mobile antenna bumper mounted would exhibit a lower radiation resistance than my installation due to cancellation of the magnetic fields produced by the auto body and the portion of the mobile antenna that is adjacent to the auto body. In other words, how the antenna is mounted is often the dominant factor determining radiation efficiency.

At 7 MHz the ATAS-120A loading coil inductance is about 60 uH giving a reactance of 2600 ohms. Let's say the Q is 40, as you say. The coil loss resistance would be 65 ohms. Add 10 ohms for ground loss and 3.5 ohms for radiation resistance and the efficiency is 4.5%. But based on two things, my coil Q figure of 90, which is backed up by the 3:1 VSWR measurement, the radiation efficiency is about 10%. In either case the radiation efficiency is nowhere near 1%.

I think you need to go read a few of Tom's articles on mobile antennas, and how they work. While the vehicle's superstructure is indeed part of the antenna system, the radiating element is the antenna itself. In this case, the length is less that 6 feet; the body of the vehicle it is mounted on notwithstanding.

Alan, Tom (W8JI) and I use the same method to model mobile antennas. We even use the same software. His 160 meter mobile antenna model and my 40 meter model are quite similar. Both models include the effects of vehicle RF current. This can be the dominating factor (and is with my installation) and that is why modelers include a vehicle model rather than placing the mobile antenna on a flat ground plane. The radiating "antenna" does indeed include the vehicle and the mobile antenna.

While true, it is not necessarily due to your assumes calculations. High mounting forces more of the return current through the body of the vehicle, rather than the lossy surface under it. The hidden item here is, however, are the magnitude of the ground losses due to the standing wave between the body and the surface. In fact, it is very common to have higher ground losses on 20 meters, than you do 40. Since you can't measure it, the only true way is to properly measure field strength. That is a big problem too, because very few amateur have the facilities to do so accurately, and that includes the so-called proof expressed by the various antenna shootouts.

Lastly, I can say without reservation, that no ATAS ever won an antenna shootout, no matter where or how it was mounted, the vehicle in use notwithstanding!

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